There is a demand for vehicles to lift payloads into space. Military, commercial and research interests have built satellites, and research experiments that are waiting to be launched into space. There is also a developing interest in stationing manufacturing plants in low earth orbit that will take advantage of absence of gravity and the pure vacuum of space. Space stations will require constant resupplying of raw materials and food and other supplies for the personnel present on the station.
Until recently almost all the U.S. launched payloads were lifted into space by means of the shuttle program. The shuttle was heavily subsidized which forced private enterprise to be non-competitive in cost of launch. NASA's space program was not able to fulfill its goals. In 1984 NASA was restricted from competing in the commercial launch field by the U.S. Commercial Space Launch Act.
Major aerospace companies returned to the commercial launch field with the Titan, Atlas and Delta. These rockets are designed for large payloads weighing up to 4,000 pounds. Since these rockets were designed for military applications they are as light as possible at the expense of cost.
Not all payloads are that heavy. There is a need to be able to place smaller payloads into low earth orbit at a reasonable cost. Large conventional rockets use either a solid propellant or a liquid fuel. Solid propellants are formed from a combustible binder such as a polyurethane containing a dispersion of oxidizer salt such as ammonium perchlorate and metal particles such as aluminum. Once a solid propellant is ignited it burns completely until all the fuel is consumed. There is no way to moderate or control the thrust or rate of burn. There are problems with burning instability developing within the motor and with separation of the solid motor element from the motor casing. Solid propellant rockets are very expensive. NASA mainly uses solid propellant rocket motors. Liquid rocket motors do offer the advantage of control of the rate of burn. However, liquid rockets utilize highly reactive hypergolic liquid fuel and liquid oxidizer. The fuel and oxidizer are usually stored outside of the engine until immediately before launch. Liquid motors require complex, expensive, heavy metering, valving and control mechanisms. Liquid rockets are hazardous and can destruct catastrophically.